In deciphering the cosmic puzzle of what the nature of dark energy is, we’re going to better learn the fate of the Universe. Whether dark energy changes in strength or sign is key to knowing whether we’ll end in a Big Rip or not. (SCENIC REFLECTIONS WALLPAPER)

You Don’t Need To Modify Gravity To Explain Dark Energy

Just because an idea is fashionable doesn’t mean it’s relevant for our Universe.

One of the greatest unsolved puzzles in all of science is dark energy. The Universe isn’t just expanding, but the expansion rate that we infer for distant galaxies is accelerating: their recession velocity speeds up from our perspective as time goes on. This was a surprise when it was discovered empirically in the 1990s, and more than two decades later, we still don’t understand where this mysterious form of energy, the most abundant in all the Universe, comes from.

While you can explain dark energy in the context of General Relativity, it’s recently become fashionable to attempt to explain dark energy by modifying gravity instead. Recently, the award-winning theoretical work of Dr. Claudia de Rham has come into focus, leading The Guardian to ask, “Has physicist’s gravity theory solved ‘impossible’ dark energy riddle?” It’s a fascinating possibility, but one that demands an appropriate level of skepticism.

Countless scientific tests of Einstein’s general theory of relativity have been performed, subjecting the idea to some of the most stringent constraints ever obtained by humanity. Einstein’s first solution was for the weak-field limit around a single mass, like the Sun; he applied these results to our Solar System with dramatic success. We can view this orbit as Earth (or any planet) being in free-fall around the Sun, traveling in a straight-line path in its own frame of reference. All masses and all sources of energy contribute to the curvature of spacetime, but we can only calculate the Earth-Sun orbit approximately, not exactly. (LIGO SCIENTIFIC COLLABORATION / T. PYLE / CALTECH / MIT)

You can imagine the Universe as a race between two contestants: the initial cosmic expansion…